Rub your cock against Winry's ass.

You turn Winry around and take your cock in your hand, rub it against the soft skin of Winry's buttocks. You press your semi-erect member against her butt crack and place your hands on her hips. You grind yourself against her ass, your cock rising to point upward. Eventually you need to place your cock vertically against Winry's butt crack. You now have a solid erection.